There is an urgent need for continuous evaluation of cardiopulmonary status in newborns and older infants. Recent developments in clinical application of transcutaneous oxygen sensors show promise of partially fulfilling this need. However, even the best current designs produce data whose correlation with the many variables determining the physiological state is uncertain. It is apparent that the parameters which control transcutaneous gas exchange are insufficiently understood. In the studies proposed here, we plan a basic investigation which will lead to an identification of the factors which control transcutaneous transfer. This involves theoretical modeling as well as in vivo and in vitro measurements. With these results, we hope to provide the basis for optimal design of skin probes for arterial blood gas measurement. This work may also lead to the use of skin probes for diverse applications such as transcutaneous measurement of anesthetic agents, as well as using multiple inert gases as probes for local perfusion and as a measure of pulmonary status.